1. Field of the Invention
The present invention relates to a photocoupler, a method for producing the same, and an electronic device equipped with the photocoupler.
2. Related Art
A photocoupler has a light emitting element and a light detecting element in an opposed arrangement. In this photocoupler, electric signals are converted into optical signals by the light emitting element. Then, the optical signals are transmitted from the light emitting element to the light detecting element, where the optical signals are converted into electric signals. The input side and the output side of this photocoupler are electrically insulated.
FIG. 12 shows an example of a conventional flat type photocoupler. FIG. 13 is a flowchart showing the process for producing the flat type photocoupler. According to this production process, a light emitting element 302 and a light detecting element 303 are respectively attached to lead frames 304, 305 by a conductive adhesive 306 (STEP. 401) In the next wire bonding step, the light emitting element 302 and the light detecting element 303 are electrically connected to the lead frames 304, 305, respectively (STEP. 402) Then, a transparent resin 307 is formed to provide an optical path between the light emitting element 302 and the light detecting element 303 (STEP. 403). Thereafter, a light blocking resin 308 is molded in order to avoid penetration of extraneous light and leakage of internal light (STEP. 404). An exterior wall of the molded light blocking resin 308 is plated (STEP. 405) to finish a photocoupler 301. This photocoupler 301 is tested for withstand voltage (STEP. 406), electric characteristics (STEP. 407) and appearance (STEP. 408). Having passed these tests, the photocoupler 301 is packed (STEP. 409) and shipped (STEP. 410).
In this photocoupler 301, when light emits from the light emitting element 302, the light is reflected at the interior wall of the transparent resin 307, thereby being incident on the light detecting element 303.
FIG. 14 shows an example of a conventional face-to-face type photocoupler. FIG. 15 is a flowchart showing the process for producing the face-to-face type photocoupler. According to this production process, a light emitting element 502 is attached to a lead frame 504 by a conductive adhesive 505 (STEP. 601) and electrically connected to the lead frame 504 by wire bonding (STEP. 602). Likewise, a light detecting element 503 is attached to a lead frame 506 by a conductive adhesive 505 (STEP. 603) and electrically connected to the lead frame 506 by wire bonding (STEP. 604). Then, the lead frames 504, 506 are positioned such that the light emitting element 502 and the light detecting element 503 are opposed to each other (STEP. 605). In this opposed state, a transparent resin 507 is formed to provide an optical path between the light emitting element 502 and the light detecting element 503 (STEP. 606) Thereafter, a light blocking resin 508 is molded in order to avoid penetration of extraneous light and leakage of internal light (STEP. 607). An exterior wall of the molded light blocking resin 508 is plated (STEP. 608) to finish a photocoupler 501. This photocoupler 501 is tested for withstand voltage (STEP. 609), electric characteristics (STEP. 610) and appearance (STEP. 611). Having passed these tests, the photocoupler 501 is packed (STEP. 612) and shipped (STEP. 613).
In this photocoupler 501, when light emits from the light emitting element 502, the light is transmitted straight through the transparent resin 507 to the light detecting element 503.
Incidentally, both types of conventional photocouplers employ a silicone resin as the transparent resin which serves as an optical path between the light emitting element and the light detecting element. A silicone resin, however, has a low viscosity in an unhardened state. Therefore, if applied too much, the silicone resin may flow along the reverse surface or terminals of a lead frame. In the worst case, the silicone resin leaks beyond the light blocking resin, degrading a photocoupler to a defective one. On the other hand, if applied too little, the silicone resin cannot provide an optical path between the light emitting element and the light detecting element, also degrading a photocoupler to a defective one. Thus, as far as the conventional techniques are concerned, it is very troublesome to control the amount of silicone resin to be applied, and extremely difficult to stabilize the characterisetics and quality of a photocoupler.